The answer is part scientific, part administrative, and part practical.

Scientifically, the spike protein is made up of many different epitopes (smaller parts of the protein that are recognized by antibodies or T cells). Some of those epitopes still convey protection for current variants.

Administratively, Covid vaccines still have to go through hoops that eg influenza vaccines currently don't, so it's easier to just use what's already gone through trials and approval processes. Soonish, the vaccines can bypass those regs and update as fast as flu vaccines do. Whether that is helpful or not is up for debate, as we've seen that flu vaccines are often outdated by the time they're released.

Practically, if the vaccines are still effective, there's not a lot of pressure on eg Moderna or Pfizer to "retool" the production lines to make an updated vaccine.

To prevent the original strains from coming back when immunity to those (eventually) wanes.

We don't want to start going backwards. Plus, there's cross-reactive immunity so that similar mutations can be recognised by the immune system without ever seeing that particular one before.

Keeping a wide breadth of spike mutations allows that to work more effectively.

Edit: u/nomnomnomnomRABIES, the reason is that Flu is an entirely different beast to COVID. Despite all the mutations COVID has gone through, it is not all that different to the original strain (which is a good reason why our immunity still holds so well). Coronaviruses do not mutate all that much, as they have the largest genome of all RNA viruses. COVID is just mutating a bunch, relatively, because of how widespread it is.

Flu, on the other hand, drifts massively, and constantly. There's no point including older strains because it doesn't help you fend off next year's Flu. Maybe once or twice in your life you'll come across a strain that is similar to one you were previously vaccinated against, which is nice, but no point wasting time cramming a Flu vaccine with all these historical Flu strains.